Origin and emplacement of ultramafic mafic intrusions in the Erro-Tobbio mantle peridotite (Ligurian Alps, Italy)

Transcription

1 Lithos 94 (2007) Origin and emplacement of ultramafic mafic intrusions in the Erro-Tobbio mantle peridotite (Ligurian Alps, Italy) Giulio Borghini a, Elisabetta Rampone a,, Laura Crispini a, Roberto De Ferrari a, Marguerite Godard b a Dipartimento per lo Studio del Territorio e delle sue Risorse, Università di Genova, Corso Europa 26, Genova, Italy b Laboratoire de Tectonophysique, ISTEEM, CNRS, Université Montpellier 2, cc 49, Place Eugène Bataillon, Montpellier Cedex 5, France Received 15 May 2005; accepted 2 June 2006 Available online 7 September 2006 Abstract The Erro-Tobbio peridotites (Voltri Massif, Ligurian Alps) represent subcontinental lithospheric mantle tectonically exhumed during Permo Mesozoic extension of the Europe Adria lithosphere. Previous studies have shown that exhumation started during Permian times, and occurred along kilometer-scale lithospheric shear zones which enhanced progressive deformation and recrystallization from spinel- to plagioclase-facies conditions. Ongoing field and petrologic investigations have revealed that the peridotites experienced, during uplift, a composite history of diffuse melt migration and multiple episodes of ultramafic mafic intrusions. In this paper we present the results of field, structural and petrologic geochemical investigations into a sector of the Erro-Tobbio peridotite unit that preserves well this multiple intrusion history. Melt impregnation in the peridotites is evidenced by significant plagioclase enrichment and crystallization of unstrained orthopyroxene replacing kinked mantle olivine and clinopyroxene; impregnating melts were thus opx-saturated. Melt rock interaction caused chemical changes in mantle minerals (e.g. Al decrease and REE increase in cpx; Ti and Cr# enrichment in spinel). Nevertheless, clinopyroxenes still exhibit LREE depletion (Ce N /Sm N = ), indicating a depleted signature for the percolating melts. Melt impregnation was thus related to diffuse porous flow migration of depleted MORB-type melt fractions that modified their compositions towards opx saturation by mantle melt interaction during ascent. The impregnated peridotites are intruded by a hectometer-scale stratified cumulate body, mostly consisting of troctolites and plagioclase wehrlites, showing gradational, interfingered contacts with the host mantle rocks. Subsequent intrusion events are revealed by the occurrence of olivine gabbros as decameter-wide lenses, variably thick (centimeter- to meter-scale) dykes and thin dykelets, which crosscut both the peridotite foliation and the magmatic layering in the cumulates. Overall, major and trace element compositions of minerals in the intrusives indicate that they represent variably differentiated cumulus products crystallized from rather primitive N-MORB-type aggregated melts. Slightly more evolved compositions are shown by olivine gabbros, relative to the troctolites and plagioclase wehrlites of the cumulate body. Mineral chemistry features (e.g. the Fo An correlation and high Na, Ti, Mg# in cpx) indicate that the studied intrusive rocks crystallized at moderate pressure conditions (3 5 kbar,i.e.9 15 km depth). Our study thus points to a progressive transition from porous flow melt migration to emplacement of magmas in fractures, presumably related to progressive change of lithospheric mantle rheology during extension-related uplift and cooling Elsevier B.V. All rights reserved. Keywords: Gabbroic rocks; Olivine cumulates; MORB-type intrusion; Mantle peridotite; Melt impregnation Corresponding author. Dipartimento per lo Studio del Territorio e delle sue Risorse, Università degli Studi di Genova, Corso Europa 26, I Genova, Italy. Tel.: ; fax: address: betta@dipteris.unige.it (E. Rampone) /$ - see front matter 2006 Elsevier B.V. All rights reserved. doi: /j.lithos

2 G. Borghini et al. / Lithos 94 (2007) Introduction Recent petrologic and isotope studies on the Ligurian ophiolites (Northern Apennines and Ligurian Alps), which are lithosphere remnants of the Jurassic Ligurian Tethys ocean, have indicated that they do not represent mature oceanic lithosphere formed at a mid-ocean ridge setting because they consist of a peculiar gabbro peridotite association, in which older (Proterozoic and Permian) subcontinental lithospheric mantle was intruded by younger (Jurassic) MORB-type magmas (Piccardo et al., 1994; Rampone et al., 1995, 1998; Rampone and Piccardo, 2000; Rampone et al., 2005). Thus, it has been inferred that the Ligurian ophiolites represent the structure and composition of oceanic lithosphere that floors embryonic oceans developed through passive continental rifting and slow-spreading oceanization (Rampone and Piccardo, 2000; Tribuzio et al., 2000a,b; Piccardo et al., 2002). This interpretation has been further supported by petrologic and radiometric studies on the Northern Apennine and Tuscan ophiolitic gabbroic rocks, which mostly occur as discrete intrusions within mantle peridotites. These studies have provided evidence that the gabbroic rocks were intruded under low- to moderate-pressure conditions (2 5 kbar), similar to some gabbroic suites from modern slowspreading ridges. Moreover, the ages of magmatic crystallization of some gabbroic rocks (EL and Tuscany; Tribuzio et al., 2004) are older than the oldest pelagic sediments (radiolarites) of the Ligurian Tethys ocean (Bill et al., 2001). In spite of a rather large number of petrologic and isotope studies devoted to both mantle peridotites and gabbroic rocks from the Northern Apennine ophiolites (Rampone et al., 1993, 1995, 1997, 1998; Tribuzio et al., 2000a,b, 2004) and to mantle peridotites from the Erro- Tobbio (ET) (Voltri Massif, Ligurian Alps) ophiolitic unit (Piccardo et al., 1992; Hoogerduijn Strating et al., 1990, 1993; Rampone et al., 2004, 2005), very few studies have yet been made on the mafic rocks associated with the Erro- Tobbio peridotites. Available studies mostly refer to late gabbroic and basaltic dykes which crosscut all mantle structures and show MORB-type affinity (Piccardo et al., 2001). Ongoing field and petrologic investigations (Borghini et al., 2004; De Ferrari et al., 2004; Piccardo et al., 2004a) have revealed that during uplift the ET peridotites experienced a composite history of diffuse melt migration and multiple episodes of gabbroic injections forming discrete intrusive bodies and decameter-scale dykes. In this paper we present an insight into a sector of the ET peridotite unit that well preserves this multiple intrusion history, by results of field, structural and petrologic geochemical investigations. Our major aim is to provide structural and petrologic constraints on the impregnation intrusion steps of the exhumation-related evolution of the ET mantle. Samples studied are plagioclase-bearing wehrlites, clinopyroxene-bearing troctolites and olivine gabbros, representative of the intrusion events. A few mantle peridotite samples, constituting the country rocks, have been also included in this study, in order to define the relationships, if any, between the diffuse melt impregnation and discrete melt intrusion events. Specific aims of this study are (i) to define the magmatic affinity of parental melts, and (ii) to provide an estimate of the conditions of intrusion of the gabbroic rocks. 2. Petrologic and structural background on the Erro-Tobbio mantle Peridotites from the ET unit (Voltri Massif, Ligurian Alps) (Fig. 1) have long been thought to be the Alpine equivalents to the Northern Apennine ophiolites, except that they were involved in the Alpine orogeny (Bezzi and Piccardo, 1971; Scambelluri et al., 1991; Piccardo et al., 1992). In spite of the Alpine overprint, the ET unit preserves kilometer-scale volumes of unaltered peridotites that retain mantle textures and mineral assemblages, thus allowing the study of their pre-alpine mantle evolution (Bezzi and Piccardo, 1971; Ernst and Piccardo, 1979; Ottonello et al., 1979; Piccardo et al., 1990; Hoogerduijn Strating et al., 1990; Vissers et al., 1991; Piccardo et al., 1992; Hoogerduijn Strating et al., 1993; Capponi et al., 1999; Borsi et al., 1996; Piccardo et al., 2004a; Rampone et al., 2004, 2005). The ET mantle peridotites consist mostly of partly serpentinized clinopyroxene-poor lherzolites and harzburgites, which commonly display spinel-bearing assemblages. Petrologic and structural investigations have documented that they record a composite tectono-metamorphic subsolidus decompressional evolution, testified by progressive reequilibration from spinel- to plagioclase- to amphibole-facies conditions, and deformation from granular to tectonite to mylonite fabrics, developed along kilometer-scale shear zones interpreted as fragments of an extensional detachment system (Hoogerduijn Strating et al., 1990; Vissers et al., 1991; Hoogerduijn Strating et al., 1993; Rampone et al., 2005). On this basis, it has been inferred (Hoogerduijn Strating et al., 1993) that the ET peridotites represent subcontinental lithospheric mantle which was tectonically denuded during rifting and opening of the Jurassic Ligurian Tethys ocean, similar to the Northern Apennine (External Ligurides) ophiolitic peridotites. However, Sm/Nd radiometric dating on plagioclase peridotites, representative of the plagioclase-bearing mylonitic extensional shear zone, have yielded ages of 273±16Ma and

3 212 G. Borghini et al. / Lithos 94 (2007) ± 16 Ma, for the plagioclase-facies recrystallization stage, thus suggesting that the onset of exhumation of the ET mantle occurred at a much earlier time than the inferred Triassic Jurassic age of rifting and opening of the Ligurian Tethys ocean (Rampone et al., 2005). Geochemical investigations have revealed that the ET mantle protoliths record a composite history of partial melting and melt migration by reactive porous flow, which presumably preceeded exhumation (Rampone et al., 2004, 2005). Ongoing field and petrologic investigations on the ET peridotites (Hoogerduijn Strating et al., 1993; Piccardo et al., 2004a; Borghini et al., 2004; De Ferrari et al., 2004;andthis study) have revealed evidence of diffuse melt percolation/ impregnation and multiple episodes of gabbroic intrusions. Melt impregnation is evidenced by the occurrence of plagioclase-rich peridotites, cut by a network of dunite channels and gabbronoritic dykelets which, in turn, crosscut the tectonite and mylonite spinel peridotite foliation; this suggests that melt impregnation and dunite formation postdates the spinel-facies lithospheric deformation (Piccardo et al., 2004a). In the studied area, plagioclase-rich peridotites are intruded by hectometer-scale body of ultramafic mafic rocks. Mantle peridotites and the ultramafic mafic body are then intruded by olivine gabbro dykes and lens-shaped bodies, thus indicating a composite history of diffuse melt impregnation and multiple episodes of discrete gabbroic intrusions. 3. Field occurrence The area investigated in this paper is situated in the southern sector of the ET unit (Fig. 1) and exposes an about 0.5 km wide body of mantle peridotites and ultramafic mafic associated intrusions, mostly recording Fig. 1. (A) Sketch map of the Voltri Massif, showing the Erro-Tobbio peridotite and surrounding units (redrawn after Federico et al., 2004). Open circle is the studied area. Inset (B) shows the location of the Ligurian ophiolites in the framework of the Northern Apennines and Western Alps.

4 G. Borghini et al. / Lithos 94 (2007) a pre-alpine mantle history (Fig. 2). Preserved mantle peridotites are bounded by serpentinites, with minor metarodingites, strongly overprinted by Alpine metamorphism and deformations. Mantle peridotites are strongly serpentinized and show a low-strain tectonite foliation defined by pyroxene shape-preferred orientation (Fig. 3A). They display a significant enrichment in plagioclase, this latter mostly occurring as interstitial Fig. 2. Structural map of the studied area.

5 214 G. Borghini et al. / Lithos 94 (2007) Gabbroic intrusions occur within both peridotites and olivine cumulates. They mostly consist of nearly isotropic medium- to coarse-grained (pegmatitic) olivine gabbros. They show a well-preserved magmatic granular texture and mineralogy which is, in places, overprinted by a highpressure metamorphic recrystallization. This recrystallization occurs in coronitic textures or in heterogeneously developed high-strain mylonitic shear zones. Alpine structures and metamorphic overprint of olivine gabbros from the ET unit have been previously described (Capponi and Crespini, 1990; Messiga et al., 1995). Olivine gabbros occur as decameter lens-shaped bodies, dykes of variable thickness (decimeter- to meterscale) and thin (centimeter-scale) dykelets, which show clear crosscutting relationships with respect to both tectonite foliation in the peridotites and magmatic layering in the cumulates. Gabbroic dykes and dykelets display rather sharp contacts with the host peridotites and olivine cumulate, and often exhibit a grain-size variability, with finer grain size toward the margins. Lenses of gabbros are elongated in a NNW SSE direction, steeply dipping towards the E, and are arranged in en echelon geometry (Fig. 2). Dykes mainly strike NNW SSE, as well. Dykelets often occur in conjugate pairs; they show a prevailing NNW SSE strike (with minor N S orientation) and dip 40 50, mainly to the east and subordinately to the west (Fig. 2). Fig. 3. (A) Pyroxenite banding crosscut by low-strain mantle tectonite foliation; (B) cumulate body apophysis (troctolite MF51) within the host mantle peridotites. blebs and veins between mantle minerals. Diffuse pyroxenite bands and meter-scale dunite bodies occur within the peridotites. Pyroxenites occur in a sub-parallel centimeter-scale banding which is crosscut at mediumhigh angle by the tectonite mantle foliation (Fig. 3A). In the northern part of the studied area, the plagioclaserich peridotites are intruded by hectometer-scale olivine cumulate body, showing variable modal composition, ranging from dunites with interstitial plagioclase and clinopyroxene, to clinopyroxene-bearing troctolites to plagioclase-bearing wehrlites. In places, this compositional variability defines a magmatic layering. In the outcrop, olivine frequently displays skeletal texture. The contact between the cumulate body and the host mantle peridotites is irregular and defined by decameter-scale plagioclase wehrlite troctolite apophyses crosscutting the pyroxenite banding and tectonite mantle foliation at low angle (Fig. 3B,). At the map scale, the contact strikes NW SE and dips towards the west. To the east, the body is in contact with serpentinites along brittle Alpine shear zones. 4. Sample description 4.1. Mantle peridotites The studied peridotites (samples MF33 and MF40) consist of partly serpentinized cpx-poor lherzolites (b10% modal clinopyroxene) showing a complete equilibration in the spinel-facies stability field. They have porphyroclastic texture and exhibit a low-strain tectonite deformation, mostly defined by undulatory extinction of tabular-shaped olivine and pyroxenes. The peridotites are characterized by the diffuse occurrence of plagioclase, in variable modal amounts (up to 10% by vol.), which is almost completely replaced by low-grade alteration products. Plagioclase occurs in peculiar microstructures clearly indicating impregnation and interaction of the peridotite matrix with percolating melt: (i) plagioclase blebs and veins between tectonite mantle olivine, (ii) unstrained orthopyroxene and plagioclase intergrowths which partially replace deformed and exsolved mantle clinopyroxene (Fig. 4A). Melt impregnation is also indicated by the presence of large undeformed poikilitic orthopyroxene grains, which display irregular, lobate contacts towards tectonite mantle

6 G. Borghini et al. / Lithos 94 (2007) Fig. 4. Mantle peridotite. (A) Orthopyroxene rim replacing clinopyroxene porphyroclast. Troctolite. (B) Euhedral olivine and chromite crystals associated with interstitial plagioclase. Plagioclase wehrlite. (C) Subhedral clinopyroxene grain including euhedral plagioclase. Group B olivine gabbro. (D) Anhedral olivine crystals associated with plagioclase. olivine; the poikilitic orthopyroxene often constitutes a single crystal in continuity with the orthopyroxene rim crystallized around mantle clinopyroxene. Such textural features indicate that the impregnating melts crystallized both plagioclase and orthopyroxene, the latter partly replacing mantle olivine and clinopyroxene Olivine cumulates The cumulate body exhibits variable modal composition (from plagioclase clinopyroxene-bearing dunite to clinopyroxene-bearing troctolite, to plagioclasebearing wehrlite) and is mostly constituted by troctolites. Samples MF8, MF15 and MF21 have been collected from the inner part of the ultramafic body, whereas sample MF51 represents an apophysis at the contact with the host mantle peridotites (see Fig. 3B). Troctolites are characterized by hypidiomorphic texture and medium- to coarse-grain size. They consist of euhedral cumulus olivine, interstitial plagioclase and subordinate poikilitic clinopyroxene (Fig. 4B). Small euhedral chromite crystals are commonly found within plagioclase and orthopyroxene occurs as discontinuous rims around olivine. Clinopyroxene also occurs as thin interstitial vermicular crystals between olivine and plagioclase. A similar texture has been observed in gabbroic rocks from oceanic (Hekinian et al., 1993; Ross and Elthon, 1997) and ophiolitic (Tribuzio et al., 1995) settings, and has been interpreted as resulting from post-cumulus crystallization of intercumulus melt. Rare pargasitic-amphibole rims are also observed, mostly confined around chromite grains. In the troctolites all minerals are unstrained, whereas in sample MF51 olivine occurs both in euhedral undeformed crystals and in large grains showing kinkbands. Based on textural features, we can reconstruct the following crystallization order: (i) olivine + chromite, (ii) plagioclase + clinopyroxene, (iii) clinopyroxene (vermicular grains) and pargasitic amphibole. The plagioclase wehrlites (samples MF47 and MF54) have been collected from apophyses within the host mantle peridotites. They are coarse-grained rocks showing hypidiomorphic texture, and consist of olivine, clinopyroxene, plagioclase and rare Cr-spinel. Olivine is

7 216 G. Borghini et al. / Lithos 94 (2007) partially serpentinized and is present both as small rounded crystals and as larger grains often showing undulatory extinction and spaced kinkbands. Clinopyroxene occurs either in subidiomorphic coarse crystals (Fig. 4C) or in interstitial and poikilitic grains. Thin interstitial vermicular clinopyroxenes between olivine and plagioclase are also observed. Plagioclase mostly occurs in anhedral interstitial grains, but small subhedral plagioclase crystals are also found within poikilitic clinopyroxene (Fig. 4C). Cr-spinel consists of small subhedral crystals mostly confined between olivine grains. The crystallization order in the plagioclase wehrlites is therefore similar to that recognized in the troctolites: (1) olivine+cr-spinel, (2) plagioclase+clinopyroxene, (3) clinopyroxene (vermicular grains) Olivine gabbros The gabbroic lenses (MF2, MF22, MF24), dykes (MF20, MF29) and dykelets (MF26) display hypidiomorphic texture and mostly consist of nearly isotropic coarse- to medium-grained olivine-bearing gabbros. In most gabbroic rocks (samples MF2, MF22 and MF26, group A) olivine (modal amount 10 15% by vol.) occurs both in euhedral grains included in clinopyroxene and/or plagioclase, and in anhedral crystals in plagioclase + clinopyroxene + olivine aggregates indicative of eutectic crystallization. Plagioclase is the most abundant mineral phase (up to 60 65% by vol.) and occurs in euhedral to subhedral crystals. Clinopyroxene (20 25% by vol.) mostly crystallizes in large anhedral grains including euhedral plagioclase. In a few gabbro samples (MF20, MF24 and MF29, group B), olivine is present in lower modal amounts (8 10% by vol.) and mostly crystallizes as anhedral crystals associated with plagioclase (Fig. 4D). In these samples, clinopyroxene occurs in poikilitic crystals which include both olivine and plagioclase. In all the studied gabbros, olivine often displays discontinuous reaction rims made by orthopyroxene, and Ti-rich pargasitic amphibole frequently occurs as a thin rim between clinopyroxene and plagioclase, or partially replacing clinopyroxene. Textural features in the gabbroic lenses, dykes and dykelets indicate a similar crystallization sequence, i.e. olivine plagioclase clinopyroxene. 5. Analytical methods Major element chemistry data of minerals in the selected samples were obtained both: (i) at the Dipartimento di Scienze della Terra, University of Genova, by means of a Philips SEM 515 electron microscope equipped with an X-ray dispersive analyzer (EDS) (accelerating potential 15 kv; sample current 20 na), and (ii) at the C.N.R. Centro di studio per la Minerogenesi e la Geochimica applicata of Firenze by means of a JEOL JXA-8600 electron microscope, equipped with four wavelength dispersive spectrometers (WDS) and one energy dispersive spectrometer (EDS) (accelerating potential 15 kv; sample current 10 na) (Vaggelli et al., 1999). In situ trace element mineral analyses were obtained by LAM-ICP-MS techniques at IGG-CNR (Istituto di Geoscienze e Georisorse) in Pavia. A detailed description of this analytical procedure is reported in Tiepolo et al. (1997). Mineral REE abundances have been normalized to the average C1 chondrite composition (Anders and Ebihara, 1982). Whole rock trace element data were determined on a quadrupole VG-PG2 Inductively Coupled Plasma-Mass Spectrometer (ICP-MS) at ISTEEM (Université de Montpellier 2, France) following the procedure described by Ionov et al. (1992). REE, Sr, Zr, Hf, Rb and Ba concentrations were determined by external calibration. Detailed description of the method and detection limits can be found in Godard et al. (2000). 6. Major element mineral chemistry Major element representative compositions of minerals from the studied peridotites, olivine cumulates and olivine-gabbros are reported in Tables Mantle peridotites In all samples, olivine shows uniform compositions and rather low forsterite contents (Fo 88 Fo 89 ) and spinels exhibit rather high Cr values [Cr/(Cr+Al) = ] coupled with high TiO 2 contents (TiO 2 = ) (Fig. 5A). Such Ti contents are much higher (at similar Cr values) compared to the compositions of spinels in the ET plagioclasereequilibrated peridotites from Rampone et al. (2005), in which plagioclase undergoes a subsolidus reaction product (Fig. 5A). Plagioclase is completely replaced by low-grade alteration products. Orthopyroxene compositions are quite homogeneous in spite of different microstructural sites (mantle porphyroclasts vs. magmatic poikilitic crystals), with Mg values [Mg/(Mg+Fe 2+ )] mostly in the range Clinopyroxenes do not display significant compositional variations from cores to rims; they are characterized by rather high Ti (TiO 2 = wt.%) and low Al (Al 2 O 3 = wt.%) concentrations (Fig. 5B) and Mg values in the range (see Table 1). In Fig. 5B, the compositions of clinopyroxene cores in the studied peridotites are

8 G. Borghini et al. / Lithos 94 (2007) Table 1 Representative major element mineral compositions of peridotites Sample MF33 MF40 cpx cpx opx opx ol sp cpx opx ol sp p.c. p.r. p.c. rim p.c. p.c. p.c. poikil. p.c. p.c. SiO TiO Cr 2 O Al 2 O Fe 2 O FeO MnO MgO CaO Na 2 O Total Mg# p.c. =porphyroclast core; p.r. =porphyroclast rim; rim=magmatic orthopyroxene rim around mantle clinopyroxene; poikil. =poikilitic crystal. compared with those of clinopyroxene porphyroclast cores in spinel and plagioclase-bearing peridotites, representative of the exhumation-related decompressional evolution of the ET mantle (Hoogerduijn Strating et al., 1993; Rampone et al., 2005). Clinopyroxene cores in the studied peridotites display systematically lower Al contents, compared to previously published data (Rampone et al., 2005) Olivine cumulates In the troctolites, olivine compositions are quite homogeneous, mostly ranging Fo 88 Fo 89.Nocompositional differences are observed in sample MF51 between euhedral and kinked olivines. Spinels display rather uniform Cr and Al concentrations (Cr# = ), whereas the Ti contents are variable (TiO 2 = wt.%). Plagioclase is characterized by variable anorthite contents in the An 59 An 69 range, without core to rim systematic variation. According to the Leake (1997) classification, amphibole rims around spinels are pargasites and pargasitic hornblendes: they display high Mg values (Mg# = ) and high Na and Ti content (Na 2 O= wt.%; TiO 2 = wt. %). Clinopyroxenes exhibit high Mg values (Mg# = ), coupled with high Al 2 O 3,Cr 2 O 3,TiO 2 and Table 2 Representative major element mineral compositions of troctolites Sample MF8 MF15 MF21 MF51 cpx plag ol sp sp cpx plag ol sp cpx cpx plag ol cpx plag ol ol core core euhed. core core core core euhed. core core verm. core euhed. verm. core euhed. kinked SiO TiO Cr 2 O Al 2 O Fe 2 O FeO MnO MgO CaO Na 2 O Total An Mg# core=core of rather large crystal; verm. =vermicular, interstitial clinopyroxene; euhed. =euhedral undeformed olivine; kinked=deformed olivine with kinkbands.

9 218 G. Borghini et al. / Lithos 94 (2007) Table 3 Representative major element mineral compositions of plagioclase wehrlites Sample MF47 MF54 cpx plag ol ol cpx plag ol ol core core euhed. kinked core core euhed. kinked SiO TiO Cr 2 O Al 2 O FeO MnO MgO CaO Na 2 O Total An Mg# core=core of rather large crystal; euhed. =euhedral undeformed olivine; kinked=deformed olivine with kinkbands. Na 2 O concentrations, consistent with the compositions of clinopyroxenes in the most primitive cumulate from oceanic and ophiolitic (e.g. Northern Apennine) settings (Fig. 6). Clinopyroxenes also show remarkable compositional variations, systematically correlated with their microstructural occurrence, i.e. TiO 2 increase and Cr 2 O 3 decrease from cores to rims of poikilitic crystals, to thin interstitial vermicular grains (Fig. 6). These compositional changes are most likely related to post-cumulus crystallization of interstitial trapped melt. Mineral compositions in the plagioclase wehrlites are similar to those described in the troctolites. Olivine displays high forsterite contents (Fo 87 Fo 88 ) in both idiomorphic and kinked grains. Plagioclases are moderately anorthitic (An 60 An 62 ) and clinopyroxenes display high Mg values (Mg# = ). As in the troctolites, their Al 2 O 3,Cr 2 O 3,TiO 2 and Na 2 O contents are similar to those of clinopyroxenes in the most primitive cumulates from oceanic and ophiolitic (e.g. Northern Apennine) settings (Fig. 6) Olivine gabbros In the olivine gabbros, clinopyroxene cores show a progressive decrease in the Mg value from group A to group B (group A: Mg#= , group B: Mg#= ) coupled with Cr and Al content decrease and Ti content increase. Overall, they display high Na concentrations (group A: Na 2 O= wt.%, group B: Na 2 O= wt.%). As shown in Fig. 6, thecompositional variations in clinopyroxenes from the olivine gabbros are consistent with chemical trends defined by Table 4 Representative major element mineral compositions of olivine gabbros (group A) Sample MF2 MF22 MF26 cpx plag ol cpx plag ol cpx plag ol core core euhed. core core euhed. core core euhed. SiO TiO Cr 2 O Al 2 O FeO MnO MgO CaO Na 2 O Total An Mg# core=core of rather large crystal; euhed. =euhedral undeformed olivine.

10 G. Borghini et al. / Lithos 94 (2007) Table 5 Representative major element mineral compositions of olivine gabbros (group B) Sample MF20 MF24 MF29 cpx plag ol cpx plag ol cpx plag ol core core anhed. core core anhed. core core anhed. SiO TiO Cr 2 O Al 2 O FeO MnO MgO CaO Na 2 O Total An Mg# core=core of rather large crystal; anhed. =anhedral olivine. oceanic gabbroic rocks from slow spreading settings (Elthon et al., 1992) which are considered to represent cumulate products of progressive magmatic differentiation. Similar compositional variations are documented in clinopyroxenes from the most primitive olivine cumulates of Northern Apennine (Tiepolo et al., 1997; Rampone et al., 1998). Plagioclase displays a progressive decrease of the anorthite content from group A (An 60 An 53 ) to group B (An 56 An 51 ); consistently, the forsterite content in olivine gradually decreases from group A (Fo 87 Fo 82 )to group B (Fo 83 Fo 80 ). Such compositional trends are shown in Fig. 7, where the gabbroic rocks, together with the olivine cumulates (troctolites and plagioclase wehrlites) define positive (forsterite anorthite) and (anorthite Mg# in clinopyroxene) correlations. Amphibole compositions are mostly pargasites to pargasitic hornblendes (according to Leake classification, 1997) with relatively high Mg values (Mg#= ), Na and Ti content (Na 2 O= wt.%; TiO 2 = wt.%). nearly flat M-to H-REE (at about C1) and weakly negative Eu anomalies (Eu N /Eu N = ) (Fig. 8A). In spite of overall high REE absolute 7. Trace element mineral chemistry Representative clinopyroxene and plagioclase trace element compositions of the studied samples are reported in Tables 6 and 7. Clinopyroxenes in the impregnated peridotites exhibit rather homogeneous trace element concentrations and do not display any significant core to rim variation. Their compositions are strongly modified when compared to already published data for clinopyroxenes in the ET spinel peridotites (Rampone et al., 2005; see Fig. 8A). They show low Sr ( ppm) and high Ti, REE, Zr, Y, Sc and V concentrations. The REE spectra are characterized by significant LREE depletion (La N /Yb N = ), Fig. 5. (A) Ti 1000 (atoms per formula units) vs. 100 Cr/(Cr+Al) of spinels from the studied peridotites; also shown are compositional field of spinels in the ET spinel and plagioclase-reequilibrated peridotites (data from Rampone et al., 2005). (B) Al 100 (a.p.f.u.) vs. Cr 1000 (a.p.f.u.) of clinopyroxene cores from the studied peridotites; also shown are the compositions of clinopyroxenes from spinel and plagioclase ET peridotites (data from Rampone et al., 2005).

11 220 G. Borghini et al. / Lithos 94 (2007) Chondrite-normalized REE patterns of clinopyroxene cores from troctolites are characterized by LREE depletion (La N /Sm N = ) and rather high M- to H-REE concentrations (at about C1; Fig. 9A). Clinopyroxenes also contain moderate amounts of other incompatible elements like Zr (13 21 ppm) and Y (17 26 ppm). Fig. 9A also shows the REE composition of an vermicular clinopyroxene from troctolite MF21, in order to illustrate the chemical changes related to post-cumulus crystallization. The vermicular clinopyroxene exhibits high M- to H- REE concentrations (up to C1),as well as high Zr (81 ppm) and Y (39 ppm) contents, although maintaining the same LREE fractionation (La N /Sm N =0.16). REE spectra of clinopyroxenes in the plagioclase wehrlites are similar to those in the troctolites (La N /Sm N = ), although displaying lower REE absolute concentrations (MREE at about 10 C1). In both troctolites and plagioclase wehrlites, plagioclases show LREE-enriched patterns (La N /Sm N = ), strong positive Eu Fig. 6. TiO 2, Cr 2 O 3, Al 2 O 3 and Na 2 O contents vs. Mg# values [100 Mg/(Mg+Fe)] of clinopyroxenes in the ultramafic mafic rocks. The fields refer to the composition of clinopyroxenes from: (i) oceanic cumulates (data from Elthon et al., 1992) and (ii) primitive cumulates from the Northern Apennine ophiolites (data from Tiepolo et al., 1997; Rampone et al., 1998). concentrations, clinopyroxenes in the impregnated peridotites still exhibit similar LREE depletion relative to clinopyroxenes in the ET spinel peridotites (Rampone et al., 2005), which can be considered representative of the mantle protoliths before the impregnation event (Fig. 8A). Fig. 7. (A) Fo [100 Mg/(Mg+Fe)] in olivine versus An [100 Ca/(Ca +Na)] in plagioclase from the studied intrusives; the sloping lines for the Mid-Cayman Rise (MCR), SW Indian Ridge (SWIR), and 26 N on the Mid-Atlantic Ridge (MAR) are approximate trends for these oceanic cumulates suites (data from Elthon et al., 1992); the grey field refers to MCR cumulates (Elthon, 1987). (B) Anorthite in plagioclase vs. Mg value in clinopyroxene from the studied intrusives, compared to different suites of oceanic (MCR, SWIR; Ross and Elthon, 1997) and ophiolitic (Northern Apennine; Tribuzio et al., 2000a) cumulates.

12 G. Borghini et al. / Lithos 94 (2007) Table 6 Representative trace element mineral compositions (ppm) of peridotites and ol-cumulates Sample Peridotite MF33 Peridotite MF40 Troctolite MF8 Troctolite MF15 Troctolite MF21 Plag-wehrlite MF47 cpx cpx cpx cpx cpx plag cpx plag cpx cpx plag cpx cpx plag p.c. p.r. p.c. p.r. core core core core core verm. core core core core Sc Ti V Sr Y Zr La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf p.c.=porphyroclast core; p.r.=porphyroclast rim; core=core of rather large crystal; verm.=vermicular, interstitial clinopyroxene; ( )=not determined. anomalies (Eu N /Eu N = ), very low HREE concentrations and rather high Sr contents (Sr = ppm). The REE spectra of clinopyroxenes in the olivine gabbros are similar to those described for clinopyroxenes in the olivine cumulates: they display analogous LREE depletion (La N /Sm N = ) and almost flat M- to H-REE in the range 8 15 C1 and are consistent with REE spectra of clinopyroxenes from the most primitive olivine cumulates of Northern Apennine (Fig. 9B). Slightly higher incompatible trace element abundances are systematically shown by clinopyroxenes in group B gabbros (HREE ranging C1, Zr=16 25 ppm; Y =21 23 ppm), relative to clinopyroxenes in group A samples (HREE at about 8 10 C1, Zr=7 10 ppm; Y=11 15 ppm). Plagioclases from both groups A and B gabbros exhibit similar LREE enrichment (La N /Sm N = ) and REE concentrations (LREE at about C1). Overall, it is remarkable that the highest LREE concentrations (up to 2 3 C1) in plagioclase are recorded by the troctolites, where plagioclase occurs as an intercumulus mineral. 8. Whole-rock trace element chemistry The trace element bulk-rock compositions of selected olivine cumulates and gabbros are reported in Table 8. Overall, they are ruled by the relative modal proportions of the cumulus minerals. Olivine cumulates display very low contents in incompatible elements like Y ( ppm), Zr ( ppm) and Sr ( ppm). They also exhibit low REE concentrations, variably flat to LREEdepleted patterns (Ce N /Sm N = ) and positive Eu anomalies (Eu N /Eu N = ). Higher M- to H-REE contents are shown by plagioclase wehrlites (2 3 C1)due to their higher clinopyroxene modal amounts, relative to troctolites (b 2 C1) mostly consisting of olivine and plagioclase (Fig. 10). Gabbroic rocks, due to higher clinopyroxene and plagioclase modal amounts, show slightly higher Zr ( ppm) and Y ( ppm) contents and significantly higher Sr concentrations ( ppm). The REE patterns are characterized by LREE depletion (Ce N /Sm N =0.48 0\56), marked Eu positive anomalies (Eu/Eu* = ) and M- to H-REE concentrations in the range 3 6 C1. Slightly higher REE abundances are shown by group B gabbros, relative to group A, consistent with their lower modal amounts of olivine (Fig. 10). 9. Discussion 9.1. Melt impregnation in the host mantle peridotites A thorough discussion of the origin and significance of the melt impregnation event is beyond the aim of this

13 222 G. Borghini et al. / Lithos 94 (2007) paper. Rather, we will briefly summarize the main microstructural and chemical features recorded by the studied peridotites, which indicate that melt impregnation most likely was a separate (earlier) and unrelated event relative to the olivine cumulate and gabbro intrusions. Field evidence indicates that mantle peridotites in the investigated area are significantly enriched in plagioclase. It mostly occurs in veins and blebs interstitial to tectonite mantle olivine or crosscutting single olivine grains, thus suggesting that its crystallization was related to melt impregnation rather than to the subsolidus spinel- to plagioclase-facies recrystallization. Peculiar microtextures like (i) the crystallization of orthopyroxene (+plagioclase) intergrowth partially replacing mantle clinopyroxene, and (ii) the occurrence of large undeformed poikilitic orthopyroxenes substituting kinked mantle olivine, indicate that the impregnation event occurred after the development of the tectonite foliation, and that the impregnating melts were in disequilibrium with the peridotite minerals. Also, they had an orthopyroxene-(silica)-saturated and clinopyroxene-undersaturated composition. Similar textural features have been ubiquitously documented in the impregnated plagioclase-rich peridotites from the Northern Apennine, Lanzo, Western Alps and Mt. Maggiore (Alpine Corsica) ophiolites (Rampone et al., 1997; Muntener and Piccardo, 2003; Rampone et al., 2004; Piccardo et al., 2004a,b; Rampone et al., 2005) and it has been inferred that such orthopyroxene-saturated impregnating melts originated at deeper mantle levels and reached orthopyroxene saturation during percolation through the overlying mantle by diffuse reactive porous flow. Mineral compositions in the peridotites have been significantly modified by interaction with the impregnating melts. When compared to published data for ET spinel peridotites which can be reasonably considered representative of the mantle protoliths before impregnation (Rampone et al., 2004, 2005), clinopyroxenes exhibit strong enrichment in Ti, M- to H-REE (Fig. 8A), Zr, Y, Sc, V, coupled to depletion in Al and Sr (see Fig. 5B). Moreover, spinels in the studied peridotites are significantly enriched in Ti, compared to available data on spinels in the ET spinel and plagioclase-reequilibrated peridotites (Fig. 5A; data from Rampone et al., 2005). It is clear however that clinopyroxenes, in spite of the observed trace element enrichment, preserve a marked LREE-depletion (La N /Yb N = ), analogous to that of clinopyroxenes in the inferred spinel peridotite protoliths. Similar mineral chemistry modifications have been documented in the impregnated peridotites from the Table 7 Representative trace element mineral compositions (ppm) of olivine gabbros Sample Ol-gabbros (group A) Ol-gabbros (group B) MF2 MF22 MF26 MF20 MF24 MF29 cpx cpx plag cpx plag cpx plag cpx plag cpx plag core core core core core core core core core core core Sc Ti V Sr Y Zr La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf core=core of rather large crystal; ( ) not determined.

14 G. Borghini et al. / Lithos 94 (2007) areas of the Erro-Tobbio Unit (Piccardo et al., 2004a; see data reported in Fig. 8B). In summary, the studied peridotites were most likely impregnated by orthopyroxene-saturated depleted melts, which originated as single depleted melt increments (produced by near-fractional melting of a MORB-type asthenospheric mantle source), and modified their composition (towards SiO 2 saturation) by upward migration via diffuse reactive porous flow. Field evidence in other sectors of the ET mantle unit (Piccardo et al., 2004a) indicate that melt impregnation postdated the Fig. 8. (A) Chondrite-normalized REE abundances of clinopyroxenes from the studied peridotites; the grey field refers to the composition of clinopyroxenes in ET spinel peridotites (from Rampone et al., 2005), which can be considered representative of the mantle protoliths before the impregnation event. (B) Chondrite-normalized REE abundances of clinopyroxenes from: (1) studied peridotites, (2) gabbronorite dykelets (data from Piccardo et al., 2004a). Computed REE abundances of clinopyroxene in equilibrium with average N-MORB (from Hofmann, 1988) are also shown. Normalization factors from Anders and Ebihara (1982). Alpine Apennine system (Rampone et al., 1997, 2004; Piccardo et al., 2004a,b). The trace element enrichment recorded by reacted (partially dissolved and replaced by orthopyroxene + plagioclase) mantle clinopyroxenes has been variably interpreted to reflect (i) equilibration with migrating melts which acquired a trace element enriched signature during upwelling via melt/peridotite interaction (i.e. pyroxene dissolution and olivine precipitation with decreasing melt volume) (Piccardo et al., 2004a,b), (ii) crystallization, within the peridotites, of trapped interstitial melts (Rampone et al., 1997) or (iii) a combination of the two processes described above (Rampone et al., 2003). In spite of such discrepancies, available studies agree that preserved LREE-depletion in mantle clinopyroxene (as observed in the studied peridotites) indicates that the impregnating melts had most likely LREEdepleted compositions, and they originated as single depleted melt fractions, rather than as aggregated MORBtype melts. Similar LREE-depleted compositions have been documented in impregnated peridotite and gabbronorite dykelets (related to the impregnation) from other Fig. 9. Chondrite-normalized REE abundances of: (A) clinopyroxenes and plagioclases in the troctolites and plagioclase wehrlites, (B) clinopyroxenes and plagioclases in the olivine gabbros compared with the compositional field defined by clinopyroxenes from the most primitive Northern Apennine cumulates (data from Tribuzio et al., 2000a), (C) melts computed in equilibrium with clinopyroxenes from olivine gabbros (using cpx/liquid partition coefficients from Suhr et al., 1998), and the average N-MORB (from Hofmann, 1988). Normalization factors as in Fig. 8.

15 224 G. Borghini et al. / Lithos 94 (2007) Table 8 Whole rock trace element compositions (ppm) of troctolites, plag-wehrlites and olivine gabbros Sample Troctolite Plag-wehrlites Ol-gabbros (group A) Ol-gabbros (group B) MF21 MF47 MF54 MF2 MF22 MF20 MF29 Rb Sr Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf spinel-facies tectonite and mylonite deformation, thus presumably occurring at rather shallow lithospheric levels, during progressive exhumation of the ET mantle. However, no clear constraints are available yet on this aspect. In terms of melt dynamics, melt impregnation in the peridotites represents a stage in which single melt increments escaped aggregation and migrated, reacted, and finally crystallized in the overlying lithospheric mantle as unmixed and isolated melt fractions. In the following, we will show that petrologic and chemical features of parental melts to the olivine cumulates and gabbros point to an unrelated, most likely subsequent, melt migration and intrusion event. values in clinopyroxene (Mg# = 89 91) indicate that they crystallized from rather undifferentiated melts (see Fig. 7 and Tables 2 and 3). In the olivine gabbros, slight but systematic compositional changes are shown by minerals from group A to group B samples, e.g. progressive decrease of the Mg values in olivine (Fo 87 Fo 80 ) and clinopyroxene (Mg# = 90 84) coupled to decrease of the anorthite content in plagioclase (An 60 An 51 ), thus indicating that the group B gabbros crystallized from relatively more evolved magmas. This is consistent with the textural observation that in group B gabbros olivine is present in 9.2. Magmatic affinity of parental melts to the intrusive rocks Overall, major element compositions of minerals in the studied intrusive rocks define narrow variations, consistent with those of rather primitive cumulates from slowspreading mid-ocean ridge settings (Elthon, 1987; Elthon et al., 1992; Ross and Elthon, 1997) (Figs. 6 and 7). Similar compositions have been also documented in gabbroic rocks intruding the Northern Apennine ophiolitic peridotites (Tiepolo et al., 1997; Rampone et al., 1998; Tribuzio et al., 2000a). The most primitive compositions are shown by troctolites and plagioclase wehrlites from the cumulate body. In these rocks, the high forsterite content in olivine (Fo 88 Fo 89 ) and high Mg Fig. 10. Chondrite-normalized whole-rock REE abundances of intrusive rocks. Normalization factors as in Fig. 8.